Squeeze composition for restoring isolation

ABSTRACT

Compositions containing an alkali swelling polymer and a pH buffer with a pH between 3 and 7 may be used to treat a wellbore. The pH buffer prevents the swelling of the alkali swellable polymer. The compositions may be placed in the wellbore, whereupon they contact another material with pH increasing properties. The pH of the compositions increases, causing the compositions to swell and form a seal. The alkali swelling polymer may be an alkali swelling latex.

FIELD OF THE INVENTION

The present invention broadly relates to well cementing. Moreparticularly the invention relates to sealant compositions comprisingalkali swellable latex as well as methods for using such compositions toservice a wellbore.

DESCRIPTION OF THE PRIOR ART

When a well such as an oil or gas well has been drilled, it is oftendesired to isolate the various producing zones from each other or fromthe well itself in order to stabilize the well or prevent fluidcommunication between the zones or shut off unwanted fluid productionsuch as water. This isolation is typically achieved by installing atubular casing in the well and filling the annulus between the outsideof the casing and the wall of the well (the formation) with cement. Thecement is usually placed in the annulus by pumping slurry of the cementdown the casing such that it exits at the bottom of the well and passesback up the outside of the casing to fill the annulus. Subsequentsecondary cementing operations may also be performed. One example of asecondary cementing operation is squeeze cementing whereby a cementslurry is employed to plug and seal off undesirable flow passages in thecement sheath and/or the casing.

The use of micro-cement slurries to block and repair the unwantedpassage of fluids through very small undesirable openings is well known(for example U.S. Pat. No. 5,127,473; U.S. Pat. No. 5,123,487; U.S. Pat.No. 5,238,064; U.S. Pat. No. 5,121,795; U.S. Pat. No. 5,125,455). It isthought the success of squeezing cement into such holes and cracks ismainly a function of the size of the hole relative to the particle sizeof the cement as well as the properties of the slurries. Also, somecement particle size distributions were defined and claimed for suchapplications. In addition the usual well cementing additives can becombined with the micro-cement to adjust the slurry properties(accelerator/retarder, thinner (dispersant), fluid-loss controladditive, defoaming agents, silica flour, lightweight additives).

While a cement slurry is one type of sealant composition used in primaryand secondary cementing operations, other non-cement containing sealantcompositions, e.g. geopolymers, may also be employed. Latex emulsions,which contain a stable water-insoluble, polymeric colloidal suspensionin an aqueous solution, are commonly used in sealant compositions toimprove the properties of those compositions. For example, latexemulsions are used in cement compositions to reduce the loss of fluidthere from and to reduce the gas flow potential of the composition asthe compositions are being pumped to the annulus. In addition, latexemulsions are used to improve the flexibility of sealant compositions.Drawbacks to using latex emulsions include a lack of sufficient strengthand elasticity.

It is difficult to squeeze cement slurry into narrow cracks or channelsin the cemented annulus and therefore optimized particle sizeddistribution cement slurries have been developed to improve injectivity.Squeeze cements as disclosed in U.S. Pat. Nos. 6,312,515 and 6,656,266are commonly used to seal narrow channels where conventional micro-finecement slurries cannot penetrate. However, the size of channels that canbe penetrated by squeeze cements is limited to approximately 5-7 timesthe size of the microcement particle size that is a channel width ofaround 80-100 microns. Therefore, a solution to squeeze fissures havinga smaller size is needed.

SUMMARY OF THE INVENTION

The invention provides a method of treating a wellbore, comprising thestep of mixing an alkali swellable polymer and a first material, whereinsaid first material does not cause said alkali swellable polymer toswell significantly; and placing said alkali swellable polymer and saidfirst material in the wellbore. Preferably, the alkali swellable polymeris an alkali swellable latex. Preferably, the first material is a pHbuffer material causing the alkali swellable polymer not to set bymaintaining the pH below a threshold. The pH buffer material containspreferably a pH decreasing material having pH decreasing properties. ThepH threshold is advantageously 4, 5, 6, 7 or 8.

In another embodiment, the alkali swellable polymer is further combinedwith a second material, said second material having pH increasingmaterial. By coupling effect of pH increasing material and pH decreasingmaterial it is possible to control setting of the alkali swellablepolymer. In this way, fracture or cracks located deeper can be treated.

In a first embodiment, the wellbore comprises a first sealantcomposition set herewith having pH increasing properties and wherein thestep of placing the alkali swellable polymer and the first material isdone in the vicinity of said first sealant composition to form a secondset sealant composition. In this way, the alkali swellable polymer isused as a squeeze composition for restoring isolation of for example afirst sealant composition which was damaged. By way of examples, thefirst sealant composition is cement or geopolymer.

In a further step, a third sealant composition is placed in the wellborein the vicinity of the first sealant composition to form a third setsealant composition before the step of placing the alkali swellablepolymer and the first material. In an alternative step, a third sealantcomposition is placed in the wellbore in the vicinity of the secondsealant composition to form a third set sealant composition after thestep of placing the alkali swellable polymer and the first material. Byway of examples, the third sealant composition is squeeze cement,microcement and geopolymer or another type of alkali swellable polymer.In this way, combination of the alkali swellable polymer and the thirdsealant are used as squeeze compositions for restoring isolation of forexample a first sealant composition which was damaged.

The method can further comprise the step of placing a third material inthe wellbore before the step of placing the alkali swellable polymer andthe first material in the wellbore. By way of examples the thirdmaterial can be a pre-flush of buffer material or other fluid (e.g.normal latex) to delay the reaction with the first sealant compositionhaving pH increasing properties.

In a second embodiment, the alkali swellable polymer forms a second setsealant composition and the method comprising further the step ofplacing a first sealant composition in the wellbore in the vicinity ofsaid second sealant composition to form a first set sealant composition.By way of examples, the first sealant composition can be squeeze cementor microcement and geopolymer. In a further step, a third sealantcomposition is placed in the wellbore in the vicinity of the firstsealant composition to form a third set sealant composition.

The step of placing an alkali swellable polymer may be done with adownhole tool lowered within the wellbore. As well, the third sealantcomposition can be placed with such or similar tool. When the wellcomprises a casing, the method can further comprises the step of forminga hole in the casing before the step of placing an alkali swellablepolymer through the hole.

As well, the step of placing an alkali swellable polymer may be done bypumping it from surface.

According to another aspect of the invention, it is disclosed a sealantcomposition for use in a wellbore comprising: an alkali swellablepolymer and a first material, wherein said first material does not causesaid alkali swellable polymer to swell significantly. Preferably, thefirst material is a pH buffer material causing the alkali swellablepolymer not to set by maintaining the pH below a threshold. The pH ofthe pH buffer material is preferably between 3 to 8, more preferablybetween 3 to 7 and most preferably between 4 to 7. Preferably, thealkali swellable polymer is an alkali swellable latex.

The sealant composition can further comprise particles having an averageequivalent diameter of less than 40 microns, wherein said particles donot degrade significantly in the well. Preferably, the particles have anaverage equivalent diameter of less than 20 microns and more preferablyof less than 10 microns. Those particles have an impact on the resistantproperties of the set sealant composition. By way of examples, thealkali swellable polymer can also contain other latexes or nanorubber ornanosilica to optimize the properties of the repairing sealant made ofalkali swellable polymer.

According to another aspect of the invention, it is disclosed a methodof treating a wellbore, wherein the wellbore comprises a first sealantcomposition set herewith having pH increasing properties and wherein themethod comprises the step of placing an alkali swellable polymer in thewellbore in the vicinity of said first sealant composition to form asecond sealant composition. Preferably, the alkali swellable polymer isan alkali swellable latex.

Preferably, the alkali swellable polymer is further combined with a pHbuffer material. In one embodiment, the pH buffer material has pHincreasing properties for the alkali swellable polymer and in anotherembodiment (not exclusive); the pH buffer material has pH decreasingproperties for the alkali swellable polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the present invention can be understood with theappended drawings:

FIG. 1 is a schematic diagram of the thickening mechanism of the alkaliswellable emulsions according to the invention.

FIG. 2 shows diagram showing the principle of sample preparation for thetests.

FIG. 3 shows a detail of the test process: end view of the cementcylinder with a channel cut into it.

FIG. 4 shows a diagram of the experimental set up.

FIG. 5 shows a chart recording of the initial injection of TYCHEM68710-00 into the slot at 25 mL/hr and the pressure spike on restart offlow after 5 minutes.

FIG. 6 shows distribution of TYCHEM 68710-00 after separation of the twocement halves (Flow direction from right to left).

FIG. 7 shows development of pressure with time on restarting flow at 25mL/hr after styrene-butadiene latex has been left stationary in the slotfor 30 minutes.

FIG. 8 shows distribution of styrene-butadiene latex after separation ofthe two cement halves.

FIG. 9 shows shear stress as a function of the shear rate for TYCHEM68710-00 for three different temperatures 25 deg C., 60 deg C. and 80deg C.

FIG. 10 shows shear stress as a function of the shear rate for VISCALEXHV30 for three different temperatures 25 deg C., 60 deg C. and 80 deg C.

FIG. 11 shows the set-up used for injection tests on examples 5 and 6.

DETAILED DESCRIPTION

The widely used non-associative synthetics thickeners are known underthe name of alkali swellable (or soluble) emulsion (ASE). Addition ofalkali to the polymer emulsion results in neutralization of thecarboxylic acid groups, generating an anionic charge at the acid sitesalong the polymer chain. The like charges repel one another resulting inswelling and uncoiling of the polymer. This extremely large increase inthe hydrodynamic volume of the neutralized ASE polymer, versus the samepolymer in its emulsion state, is responsible for a significant build incompound viscosity, at relatively low polymer concentration. Aqueousswellable emulsions are high molecular weight polymers. They are liquidproducts generally commercialized at low concentration of 25-30% of drymatter, easily soluble, and pH dependent, able to provide excellentlow-shear rate viscosity, gel structure, sag resistance and stability ontime; and increasing yield value, settling and sag resistance,resistance to viscosity drop when diluting, microbial attack.

Alkali swellable emulsion rheology modifiers are often based onhomopolymers of (meth)acrylic acid and copolymers of (meth)acrylic acid,(meth)acrylate esters, and maleic acid, among many others, Table 1 is aview of ASE chemistry.

TABLE 1

(R = CH₂ or OH)

Also hydrophobically modified alkali swellable emulsions (HASE) existe.g. Akcogum SL920.

FIG. 1 is a schematic diagram of the thickening mechanism. Preferably,the alkali swellable polymers are based on latexes: alkali swellablelatex (ASL). They are designed with a well-controlled particle sizedistribution (typically <200 nanometers) that allows it to have lowviscosity at high solids concentration. The very low viscosity and smallparticle size allows a high injectivity through small features. The highsolid content will provide high gel strength when swelled at high pH.Without limitation, examples of suitable commercially available alkaliswellable latexes include TYCHEM 68710-00 of Dow chemical; ACRYSOL U 615of Rohm and Hans; SN THICKENERs 920, 922, 924, 634 and 636 of San NapcoLimited, Sanyo Chemical Industry, Japan; ALCOGUM SL-120, SL920 of AlcoChemical, a National Starch Company; HEUR-ASE P206 of Dow ChemicalCompany; ADCOTE 37-220 of Robin and Haas Company; and JETSIZE AE-75 ofEka Chemicals. TYCHEM 68710 is a carboxylated styrene/butadienecopolymer supplied as a ˜35% by weight aqueous emulsion; VISCALEX HV30of Ciba Specialty Chemicals.

Current invention is made to be used in a wellbore of the type oil, gas,water including geothermal wells but also carbon dioxide storing wellsor injection wells. Such wells comprise as said a first sealantcomposition, which for example is a cement, a geopolymer, or a mixturethat can viscosify in wellbore zones where a fluid (e.g., drillingfluid) is being lost. For instance, the sealant composition mayviscosify in a loss-circulation zone and thereby restore circulation.The viscosified mixture can set into a flexible, resilient and toughmaterial, which may prevent further fluid losses when circulation isresumed. Said first sealant composition has pH increasing properties,i.e. a compound capable of increasing the pH of the first sealantcomposition to about 7 or higher.

According to the method of the invention, the alkali swellable polymerpreferably an alkali swellable latex and a pH buffer material having pHdecreasing properties for the alkali swellable polymer is placed in thewellbore. The alkali swellable polymer can also contain other latexes ornanorubber or nanosilica.

The pH buffer material having pH decreasing properties is made ofacid-producing material which includes any compound capable ofgenerating hydrogen ions (H⁺) in water to react with or neutralize abase to from a salt. Examples of suitable acid-producing materialsinclude without limitation organic acids as e.g. citric acid, aceticacid, formic acid, mineral acids as e.g. carbonic acid, partiallyneutralized salts e.g. K₂HPO₄, KH₂PO₄ . . . .

The pH buffer material can also have pH increasing properties. Ininstance, the pH buffer material is made of a base-producing materialwhich includes any compound capable of generating hydroxyl ions (OH⁻) inwater to react with or neutralize an acid to from a salt. Examples ofsuitable base-producing materials include without limitation ammonium,alkali and alkali earth metal carbonates and bicarbonates, alkali andalkali earth metal hydroxides, alkali and alkali earth metal oxides,alkali and alkali earth metal phosphates and hydrogen phosphates, alkaliand alkaline earth metal sulphides, alkali and alkaline earth metalsalts of silicates and aluminates, water soluble or water dispersibleorganic amines, polymeric amines, amino alcohols, or combinationsthereof.

Preferably, said base-producing material can be encapsulated with atleast one encapsulating material. The base-producing compound may beencapsulated to delay its reaction with the alkali swellable polymer andto postpone the formation of a higher viscosity swollen polymer product.It is to be understood that the base-producing material can be a liquidfor example an aqueous solution or an organic liquid, or a solid. If thebase-producing material comprises an aqueous solution, it may beencapsulated in a particulate porous solid material. The particulateporous solid material comprises any suitable material that remains dryand free flowing after absorbing the aqueous solution and through whichthe aqueous solution slowly diffuses. Examples of particulate poroussolid materials include but are not limited to diatomaceous earth,zeolites, silica, expanded perlite, alumina, metal salts ofalumino-silicates, clays, hydrotalcite, styrene divinylbenzene basedmaterials, cross-linked polyalkylacrylate esters, cross-linked modifiedstarches, natural and synthetic hollow fibers, porous beads such asperlite beads, or combinations thereof. If the base producing materialis an organic liquid, it may also be encapsulated in hydrophobicallymodified porous silica in addition to the afore-mentioned absorbents.

In another embodiment, the acid-producing material can also beencapsulated with at least one encapsulating material as disclosedabove.

In alternative embodiments, encapsulation further includes an externalcoating of a polymer material through which an aqueous solution diffusesand that is placed on the particulate porous solid material. Theexternal coating can be added to further delay the reaction. Examples ofexternal coatings include but are not limited to EDPM rubber,polyvinyldiehioride, nylon, waxes, polyurethanes, cross-linked partiallyhydrolyzed acrylics, cross-linked latex, styrene-butadiene rubber,cross-linked polyurethane and combinations thereof.

In other embodiments, when the acid or base-producing compound comprisesa solid, it can be encapsulated by spray coating a variety of materialsthereon, including but not limited to a wax, a drying oil such as tungoil and linseed oil, a polyurethane, a crosslinked partially hydrolyzedpolyacrylic, a styrene-butadiene latex, a water degradable compound orpolymer, or combinations thereof.

Preferred alkali swellable polymer and alkali swellable latex weredisclosed above. In other embodiments, the alkali swellable latex maycontain crosslinking agents that are suitable for facilitating theformation of a resilient rubbery mass, which may be used during thepolymerization stage of the monomers or added to the latex prior to use(for example to the sealant composition).

In a first embodiment, the wellbore comprises a first sealantcomposition preferably a hydraulic cement or geopolymer set herewithhaving pH increasing properties (intrinsically or by adding pHincreasing material). The alkali swellable polymer is placed in thevicinity of said first sealant composition to form a second sealantcomposition. By in the vicinity of, it is understood that the alkaliswellable polymer and the first sealant composition are put in contact.It is believed that the pH increasing material embodied with the firstsealant composition reacts with acidic groups in the alkali swellablepolymer and thereby increases its viscosity along with that of theresulting second sealant composition.

In a second embodiment, when the alkali swellable polymer preferably analkali swellable latex is placed to form a second sealant composition,the method further comprise the step of placing a first sealantcomposition preferably a hydraulic cement or geopolymer to form a firstsealant composition set.

In a third embodiment, the wellbore comprises a first sealantcomposition preferably a hydraulic cement or geopolymer set herewithhaving pH increasing properties (intrinsically or by adding pHincreasing material). A third sealant composition preferably a squeezecement is placed in the vicinity of said first sealant composition e.g.cement or geopolymer to form a third sealant composition set. After, thealkali swellable polymer is placed in the vicinity of said first andthird sealant compositions to form a second sealant composition.

In a fourth embodiment, the wellbore comprises a first sealantcomposition preferably a hydraulic cement or geopolymer set herewithhaving pH increasing properties (intrinsically or by adding pHincreasing material). The alkali swellable polymer is placed in thevicinity of said first sealant composition to form a second sealantcomposition. After, a third sealant composition preferably a squeezecement is placed in the vicinity of said first and second sealantcompositions to form a third sealant composition set.

In a fifth embodiment, the method further comprises the step of placingmultiple alkali swellable polymer preferably alkali swellable latex toform multiple layers of sealant composition.

According to another method of the invention, the alkali swellablepolymer preferably an alkali swellable latex is placed in the wellborein the vicinity of the first sealant composition to form a secondsealant composition. The first sealant component has pH increasingproperties e.g. cement. The cement composition can include hydrauliccements. In some embodiments, the hydraulic cements set and harden byreaction with water. The hydraulic cements can be composed of calcium,aluminum, silicon, oxygen, sulfur, or combinations thereof. Withoutlimitation, examples of suitable hydraulic cements include Portlandcements (e.g., classes A, C, G, and H Portland cements), pozzolanacements, gypsum cements, phosphate cements, high alumina contentcements, silica cements, high alkalinity cements, Magnesia cements, andcombinations thereof. Suitable median cement particle sizes are in the 1to 200 microns range, alternatively 5 to 150 microns, and alternatively10 to 120 microns range.

In one embodiment, the pH of the second sealant composition is increasedat a desired time by using an encapsulated base having a suitablecoating composition for delayed release, which may be triggered byconditions such as changes in temperature, mechanical stresses or thechemical makeup of the final downhole mixture. The first sealantcomposition can further comprise additives for improving or changing itsproperties. Examples of suitable additives include fluid absorbingmaterials, particulate materials, superabsorbers, viscosifying agents,non-alkali swellable latexes, or combinations thereof. In an alternativeembodiment, the first sealant composition is a compressible sealantcomposition comprising foaming surfactants and foam stabilizingsurfactants.

EXAMPLES

Tests were prepared to simulate a fissure in a cement sheath to comparezonal isolation. FIG. 2 shows a diagram of the sample preparation. Aconventional 1890 kg/m³ class G cement system was prepared and cured for3 days at 60° C. in a cylindrical mould. A 37 mm diameter cylinder wasthen cored from the mould and subsequently cut lengthways (FIG. 2A). Thewidth of the saw cut is approximately 2 mm. The two half cylinders werethen placed with their flat faces together (FIG. 2B); the missing widthdue to the saw cut means that the cross section is now not perfectlycircular. The assembly B was embedded in plaster to firmly hold the twopieces together. Once the plaster set a 25 mm diameter core plug was cutfrom the sample so that the split was in the middle of the resultantcore and the cross section of the 25 mm diameter core was perfectlycircular (FIG. 2C)—there was no “missing” part due to a saw cut. Twosamples were prepared this way with lengths approximately 7 cm.

The test consists of a small slot that is filed into one flat face ofone of the cylinders to provide a channel the length of the sample asshown in FIG. 3. The assembly is then inserted in the rubber sleeve ofthe Hassler cell.

FIG. 4 shows the equipment setup for the experiment. The pump is aPharmacia model P-500 HPLC pump. The Hassler cell is from Temco modelDCH0-1.0 with a working pressure of 34 MPa. The confining pressure pumpis an Ametek Portable Hydraulic Pressure Tester, model T620. P1 is ananalogue pressure gauge to give an indication of the confining pressure.P2 and P3 were Validyne pressure transducers with CD23 signalconditioners connected that were connected to a Kipp and Zonen chartrecorder. P3 was a 10 psi full scale transducer that could be isolatedfrom the system by a valve when pressures increased. P2 was a 200 psipressure transducer that could indicate up to 400 psi via the CD23signal conditioner. The pressure transducers were calibrated against anAmetek Jofra Instruments PPCE pressure calibrator. The displacementcylinder (no reference) was used so that latex would not be pumpedthrough the HPLC pump. When required the cylinder was filled with latexand water pumped into the top by the HPLC pump to displace the latexthrough the cement sample. The cylinder was bypassed when pumping wateronly. There would be a little dilution of the latex at the top latexwater interface, but the cylinder was never completely emptied so thedilution did not affect the results.

The test procedure was as follows: loading of the test sample into cell;applying 3 MPa confining pressure (this was increased to 6 MPa for someof the tests where high (>1.5 MPa) injection pressures were used;flowing water at different rates and measure pressures; isolatingpressure transducer P3 if necessary; adding latex to the displacementcylinder and starting pumping through the core; monitoring pressure;stopping pumping for a given time; restarting pumping and determiningmaximum pressure obtained; and repeat the pumping-stop-pumping loopseveral times.

Three lattices were used: TYCHEM 68710-00, a styrene-butadiene latex andALCOGUM SL920. TYCHEM 68710-00 is a carboxylated styrene-butadienecopolymer latex, TYCHEM 68710-00 is supplied from Dow Chemical.Styrene-butadiene latex is not an ASL and styrene-butadiene latex issupplied from Schlumberger. ALCOGUM SL920 is an alkali-swellable latex(ASL) from Alco Chemical.

Initially water was flowed through the channel to determine theeffective height using the equation for flow of a Newtonian fluidthrough a slot:

$\begin{matrix}{s = \sqrt[3]{\frac{12\mu\; L\; Q}{\Delta\; P\; w}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where: s is the channel height (m); μ is the fluid viscosity (Pa·s); Lis the length of the channel (m); Q is the flow rate (m³/s); ΔP is thepressure drop across the sample (Pa); w is the width of the channel (m).

For example 1 the average width of the engraved channel was measured andthe average height of the channel calculated from equation 1 and thewater flow measurements. With the channel height estimated from waterflow, Equation 1 is used to determine the flowing viscosity of latexthrough the sample. This will give an average viscosity of the flowingfluid as the latex viscosity will depend on the time it has spent incontact with cement. To estimate the yield stress of the fluid onrestarting flow the maximum pressure measured multiplied by the channelcross section area was set equal to the yield stress multiplied by thecontact area of the channel (top and bottom). The effect of the sidewalls was ignored. This gives:

$\begin{matrix}{{{Yield}\mspace{14mu}{stress}} = \frac{P\; s\; w}{2L}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Example 1

The calculated slot height determined from water flow at 3 differentrates is shown in Table 2. The estimated slot height is 61 microns asdetermined from the two highest flow rates. The low flow rate gives aslightly higher slot height but this is due to more significant errorsin the pressure measurement at low differential pressures.

TABLE 2 Flow rate Calculated slot height (ml/hr) Pressure (psi) (μm)from equation 1 400 6.4 61 200 3.2 61 100 1.1 69

Following water injection Tychem 68710-00 was pumped through the channelin the cement cores. The pressure recorded across the sample is shown inFIG. 5—the first curve (left) is the high pressure transducer (1.4 MPafull scale) and the second curve (right) is the low pressure transducer(0.07 MPa full scale). On initial flow the ASL flows into the slot andthe first volume is exposed to the clean cement and the consequent pHincrease causes the viscosity of the system to increase. The subsequentvolumes of ASL are exposed to cement that has already been in contactwith latex so the pH increase is lower. The pressure trace shows thiswith an increase in pressure initially (corresponding to ASL filling theslot) and a subsequent decrease in pressure as the first volume of ASLexits the slot. Flow was not continued long enough for equilibrium to bereached. The flow was stopped for 5 minutes before restarting at 25mL/hr. A large pressure spike (150 psi) is observed prior to a rapiddecrease. During the 5 minute stationary period the pH of the ASL hasincreased due to prolonged contact with the cement. This has increasedthe yield stress of the fluid thus requiring a high pressure to initiateflow. However, once the fluid starts to move there is a rapid decreasein pressure as the fluid that has been in contact with the cement isdisplaced by fresh ASL. At 25 mL/hr the volume of the slot is displacedin 6 seconds. The decrease in pressure takes much longer than thisindicating that not all the high pH ASL is displaced but that there is alayer on the walls of the slot that takes some time to remove.

The flow was subsequently stopped for 30 minutes and the pressure spikerecorded after restarting flow at 25 mL/hr. This process was repeated acouple of times. The yield stress, calculated from the pressure spike,and the viscosity, calculated from the flowing pressure drop, are shownin Table 3.

TABLE 3 Yield Viscosity Pressure Peak Stress (mPa · s) (MPa) (Pa)Initial peak pressure during flow — 0.03 — Restart after 5 minutesstationary — 1.0 430 Equilibrium flow at 25 mL/hr  90 — — Restart after30 minutes stationary — >1.4 >570 Equilibrium flow at 25 mL/hr 100 — —Restart after 30 minutes stationary — 2.6 1100

The results show that when the fluid flows its viscosity is low allowingit to penetrate into cracks even when in contact with a high pHmaterial. However, when left stationary for a few minutes the yieldstress increases to a high level thus requiring a high pressure toinitiate flow. These are characteristics of a system that could be usedto repair cracks or other small defects in cement sheaths. The cementsystem was removed from the Hassler cell and the two parts separated.The distribution of the ASL in the slot is shown in FIG. 6. It is clearthat the ASL has flowed through the well defined slot. The distributionof ASL on both halves is complementary.

The two cement halves were cleaned to remove the ASL and the slotre-engraved to ensure fresh surfaces were available. The calibrationmeasurements were performed with water on the slightly modified slot.The re-engraved slot had an average height of 86 microns. Followingcalibration of styrene-butadiene latex was injected into the slot as acontrol measurement. Styrene-butadiene latex is not an alkali-swellablelatex so it would not be expected to behave in the same way as an alkaliswellable latex. A pressure plot recorded on restarting flow withstyrene-butadiene latex after a 36 minute stationary period is shown inFIG. 7. The maximum pressure attained was 0.014 MPa corresponding to ayield stress of 8 Pa. The distribution of the styrene-butadiene latexafter removal of the cement plug from the Hassler cell and separation ofthe two halves is shown in FIG. 8. The styrene-butadiene latex hasremained very liquid hence it has covered almost all the core surface,not just the channel.

Alkali swellable lattices can be injected at relatively low pressuresinto narrow cracks. Micro-cement slurries cannot be injected into cracks60 microns in width. When stationary and in contact with a substrate athigh pH the latex develops a high yield stress. This high yield stresswill prevent subsequent migration of fluids through cracks in the cementsheath. Conventional lattices do not show the same behaviour. Alkaliswellable lattices can be used as sealants to repair leaks due to narrowchannels or cracks in or close to cement sheaths.

Example 2

Example 2 is performed as before except that once prepared the cementsample was stored in water at room temperature for months until the testwas performed. This test will simulate the situation in a wellbore afterthe first sealant has been in place for some time.

The calculated slot height determined from water flow at 4 differentrates is shown in Table 4. The estimated slot height is 61 microns asdetermined from the two highest flow rates. The low flow rate gives aslightly higher slot height but this is due to more significant errorsin the pressure measurement at low differential pressures.

TABLE 4 Flow rate (ml/hr) Pressure (psi) Calculated slot height (μm) 4009.2 55 300 6.9 55 200 4.5 56 100 2.1 57

Following water injection Alcogum SL920 was pumped through the channelin the cement cores. The initial flow at 100 mL/hr gave a pressure dropacross the sample of 13.9 psi. The flow was stopped for 5 minutes andthen restarted at 25 ml/hr. There was no noticeable pressure increasewhen the flow was restarted. The flow was stopped for 32 minutes beforerestarting the flow at 25 mL/hr. The pressure built up graduallyreaching 200 psi after 26 minutes of flow. The measurement was thenstopped as the limit of the pressure transducer was reached.

The Alcogum SL920 could be pumped into a narrow crack with a relativelylow injection pressure. Once the product was left stationary for aperiod of 30 minutes there was sufficient interaction with the cement tocause a significant increase in viscosity. When flow was restarted thepressure built up to 200 psi. The Alcogum was able to block the crack inthe cement, but the behaviour was slightly different from the Tychemproduct; the pressure built up more slowly during injection but theproduct was able to withstand a higher pressure.

Other tests were realized with two lattices: TYCHEM 6870-00 and VISCALEXHV30. TYCHEM 68710-00 is supplied from Dow Reichold and VISCALEX HV30 issupplied from Ciba Specialty Chemicals. The first one is based on acarboxylated styrene-butadiene copolymer, while the second one is awater dispersion of anionic polyacrylate copolymers. Some of theirproperties are reported in Table 5.

TABLE 5 TYCHEM VISCALEX 68710-00 HV30 Polymer type Styrene- Acrylatebutadiene pH 4.5 3 Non Volatile 34 30 content, %

Example 3

To prove the high injectivity of alkali swellable lattices rheologymeasurements have been performed at 25 deg C., at 60 deg C. and at 80deg C. using a standard oilfield viscometer.

The values of shear stress as a function of shear rate, found for TYCHEM68710-00, are plotted in FIG. 9, while those obtained for VISCALEX HV30are represented in FIG. 10. Small differences are observed when thetemperature is increased. In Table 6 the plastic viscosity, Pv, and theyield stress, Ty, obtained by applying the Bingham model are reportedfor different temperatures. The values are considerably lower than thoseobtained with standard cement systems.

TABLE 6 Temperature 25 deg C. 60 deg C. 80 deg C. TYCHEM 68710-00 P_(v)(cP) 11 7 11 Ty (lbm/100 ft²) <2 <2 2.1 VISCALEX HV30 P_(v) (cP) 14 10 9Ty (lbm/100 ft²) 3.0 <2 2.7

Example 4

In order to test the ability of ASL based fluids to penetrate in narrowgaps injection tests have been performed by using a special set-up,shown in FIG. 11. This system is composed of a transparent Plexiglasplate (1) placed on a filter paper (2) supported by a porous plate (3).Small clamps, not shown in the figure, are used to hold the platestogether. A channel is obtained between the transparent plate and thefilter paper by using spacers of well-defined thickness (4). To estimatethe ability of the fluids to penetrate in narrow fractures the thicknessof the gap can be adjusted. The fluid is injected in the narrow channelwith a syringe pump (5) through a hole (6) on the Plexiglas plate. Thespeed of injection is 5 mL/min. After injection, the distance traveledby the fluid inside the gap is measured to determine its ability topenetrate. The maximum distance, defined by the size of the plates, is23 cm. All the tests have been performed at room temperature.

Example A

To test the swelling of ASLs in contact with a basic environment thefilter paper was wet with solutions of sodium hydroxide to simulate aporous surface with basic pH. For these experiments TYCHEM 68710-00 wasused.

When the pH is lower than 12 the ASL swells slowly; it can be injectedwell in gaps narrower than 60 microns. The length of the channel it canpenetrate is >23 cm. When the pH is higher than 12 the ASL swells fasterand cannot reach the end of the channel. In a 100 microns gap the ASLtravels a distance between 15 cm and 18 cm.

Buffers at different pH have been added to the ASL to retard theswelling and thus favour the penetration in channels with high pHsurfaces. In all these experiments the height of the channel was fixedto 100 microns, and the filter paper at the surface of the gap was wetwith a solution at pH 13. For these experiments both TYCHEM 68710-00 andVISCALEX HV30 have been used. The results obtained are summarized inTables 7 and 8.

Example B

Two buffer solutions at pH 7 and pH 5 were prepared by mixing a 0.1Msolution of citric acid and a 0.2M solution of Na₂HPO₄. The buffersolutions were then added to the ASLs in ratio 10/90 and the fluidobtained was injected in the 100 microns channel. The addition of the pHbuffers retards the swelling and the distance that the ASLs canpenetrate is increased. For the lower pH buffer the two fluids travelthrough the whole channel (>23 cm).

Example C

In a different test the buffer solutions at pH 7 or at pH 5 were firstinjected in the 100 microns channel. Successively the ASL were injectedin the gap. The swelling is retarded and the ASLs can reach a longerdistance. Again for the lower pH buffer the fluids travel on the totallength of the channel.

TABLE 7 TYCHEM 68710-00 Fluid composition Distance traveled 100% Tychem15-18 cm 90% Tychem-10% pH 7 21-22 cm Buffer 100% Tychem after injecting20 cm a pH 7 Buffer 90% Tychem-10% pH 5 >23 cm Buffer 100% Tychem afterinjecting >23 cm a pH 5 Buffer

TABLE 8 VISCALEX HV30 Fluid composition Distance traveled 100% ViscalexHV30 12-14 cm 90% Viscalex HV30-10% pH 7 18-21 cm Buffer 90% ViscalexHV30-10% pH 5 >23 cm Buffer 100% Viscalex HV30 after >23 cm injecting apH 5 Buffer

Example 5

In order to test the possibility to mix the ASLs to different materials,TYCHEM 68710-00 was blended to a styrene-butadiene latex (SB Latex) withparticle size lower than 165 nm, non-volatile content 50% and pH 10. Theweight ratio between the ASL and the latex was 50/50. Injection testswere conducted with the set-up of FIG. 14. The height of the channel was100 microns and the filter paper at pH 13.

First the blend was injected without the addition of any pH loweringfluid. The distance traveled was 9 cm. Successively pH buffers or acidicsolutions were either injected first or mixed to the blend. The resultsobtained are shown in Table 9.

When 10% of a solution 0.1M of citric acid was added to the blend thedistance reached in the channel was 22 cm. This was increased to morethan 23 cm when 10% of buffer solution at pH 5 was added.

TABLE 9 Blend composition Distance traveled 50% TYCHEM-50% SB Latex 7-9cm 50% TYCHEM-50% SB Latex 11 cm after injecting a pH 7 buffer 50%TYCHEM-50% SB Latex 15-18 cm after injecting 0.1M citric acid 45%TYCHEM/45% SB Latex/ 22-23 cm 10% citric acid 0.1M 45% TYCHEM/45% SBLatex/ >23 cm 10% pH 5 buffer

Examples 4 and 5 show that the distance that a fluid containing an ASLpenetrates in a very narrow channel in contact with a pH increasingmaterial, can be adjusted by making blends with other materials, or byusing a pH decreasing material like a pH buffer or an acid solution.This can be mixed to the ASL or injected first in the fracture to retardthe swelling of the ASL.

The invention claimed is:
 1. A method of treating a wellbore, comprisingthe step of: mixing an alkali swellable polymer and a pH buffermaterial, wherein the pH buffer material has a pH between 3 and 7 anddoes not cause said alkali swellable polymer to swell; and placing saidalkali swellable polymer and said pH buffer material in the wellbore. 2.The method of claim 1, wherein the pH buffer material causes the alkaliswellable polymer not to set by maintaining the pH below a threshold. 3.The method of claim 1, wherein the alkali swellable polymer is furthercombined with a second material, said second material having pHincreasing properties.
 4. The method according to claim 1, wherein thewellbore comprises a first sealant composition set herewith having pHincreasing properties and wherein the step of placing said alkaliswellable polymer and said pH buffer is done in the vicinity of saidfirst sealant composition to form a second sealant composition.
 5. Themethod of claim 4, comprising further the step of placing a thirdsealant composition in the wellbore in the vicinity of said firstsealant composition to form a fourth set sealant composition before thestep of placing said alkali swellable polymer and said pH buffer.
 6. Themethod of claim 4, comprising further the step of placing a thirdsealant in the wellbore in the vicinity of said second sealantcomposition to form a fourth set sealant composition after the step ofplacing said alkali swellable polymer sand said pH buffer.
 7. The methodof claim 5 wherein the third sealant composition is squeeze cement ormicrocement and geopolymer.
 8. The method of claim 1, comprising furtherthe step of placing a third material in the wellbore before the step ofplacing said alkali swellable polymer and said pH buffer in thewellbore.
 9. The method according to claim 1, wherein the alkaliswellable polymer is an alkali swellable latex.
 10. The method of claim1, wherein the step of placing an alkali swellable polymer is done witha downhole tool lowered within the wellbore.
 11. The method of claim 10,wherein the well comprises a casing, and the method further comprisesthe step of forming a hole in the casing before the step of placing analkali swellable polymer.
 12. The method of claim 1, wherein the step ofplacing an alkali swellable polymer and the first material is done bypumping said alkali swellable polymer and said first material fromsurface.
 13. A method of treating a wellbore comprising a first setsealant composition comprising squeeze cement, microcement orgeopolymers set herewith, comprising: mixing an alkali swellable polymerand a pH buffer material whose pH is between 3 and 7, wherein said pHbuffer material does not cause said alkali swellable polymer to swell;and placing said alkali swellable polymer and said pH buffer material inthe vicinity of the first set sealant composition in the wellbore;wherein said first set sealant composition has pH increasing propertiesthus causing the alkali swellable polymer to swell to form a second setsealant composition.
 14. A sealant composition for use in a wellborecomprising: an alkali swellable polymer and a pH buffer material whosepH is between 3 and 7, wherein said pH buffer does not cause said alkaliswellable polymer to swell.
 15. The sealant composition of claim 14,wherein the pH buffer material causes the alkali swellable polymer notto set by maintaining a pH below a threshold.
 16. The sealantcomposition of claim 14 further comprising particles having an averageequivalent diameter of less than 20 microns, wherein said particles donot degrade in the well.
 17. The sealant composition of claim 14,wherein the alkali swellable polymer is an alkali swellable latex.